Bigload -
I have a 15" Rythmic with dual passive radiators and the 370W PEQ amp. When I first got it, I had issues like you described, I seemed to have to turn the thing way up on the gain. Two things fix this really easily.
1. Turn up the LFE or Sub gain offset in your surround processor. In my Outlaw 990 I can trim it up 10dB, and in my research on the issue, I found a link where it said most LFE effects were pressed to disc down 6-10 dB, so this is not uncommon.
2. Brian Ding at Rythmik recommended I get a line splitter and use both sub inputs on the amp instead of a single. The amp treats this as a summer, and you get another 3-6dB of signal gain (I can't remember which).
Having done both of the above myself, the sub works awesome. I have the gain knob around 40-50% and I get as much output as I like. There's still alot left in the pedal if I desired more. Also, Brian designed the "auto-on" feature to be not so sensitive, so the above steps make it work great since you have so much signal coming into the amp.
Note that for 2-channel listening, I have no such trims. I go directly from CD player to Parasound 2100 preamp to sub, and it both "auto-on" and plays at fine levels.
If the 600W amp had been available at the time, I probably would have purchased it because I like to have the best available. But as a user of the amp, I couldn't tell you that the 370W is inadequate, because it works great in my home using the same speakers as you have. My room is roughly 33x18, and opens to an adjacent room roughly 30x15.
Note: I have no experience with Rythmik subs outside of the passive radiator version I have. What I said above may not be true for sealed subs.
Note 2: The Oppo Blu-Ray player also has LFE/sub trims for its output as well. So there is another option for trimming the system if you don't want to do it in the processor.
